For example, in wireless devices, such as base stations, user terminals, and the like in wireless communication systems, amplifiers (power amplifier; hereinafter, sometimes referred to as “PAs”) that amplify electrical power of transmission signals are provided. In the wireless devices, in general, in order to enhance the power efficiency of the PAs, the pas are operated in the vicinity of the saturation region of the PAs. However, if a PA is operated in the vicinity of the saturation region, nonlinear distortion is increased. Thus, in order to reduce electrical power leaking into an adjacent channel by suppressing the nonlinear distortion generated in the PA, a distortion compensation unit that compensates nonlinear distortion generated in the PA is provided in the wireless device.
There is a “pre-distortion (hereinafter, sometimes referred to as a “PD”) system” as one of the distortion compensation systems used in a distortion compensation unit. By previously multiplying a distortion compensation coefficient that has the inverse characteristic of the nonlinear distortion generated in the PA by a transmission baseband signal that has not been input to the PA, the distortion compensation unit that is used in the PD system increases the linearity of output signals of the PA and suppresses the distortion generated in an output signal of the PA. By multiplying the distortion compensation coefficient by the transmission baseband signal, the distortion of the transmission baseband signal is compensated. In a description below, the signal obtained by multiplying the distortion compensation coefficient by the transmission baseband signal is sometimes referred to as a “pre-distortion signal (PD signal)”. Thus, before the PD signal is input to the PA, the FD signal becomes a previously distorted signal in accordance with the inverse characteristic of the nonlinear distortion in the PA.
For example, as a distortion compensation unit in a PD system, there is a unit that includes a look-up table (hereinafter, sometimes referred to as a “distortion compensation table”) in which a plurality of distortion compensation coefficients is stored. The distortion compensation unit that includes the distortion compensation table reads, from the distortion compensation table, a distortion compensation coefficient that is in accordance with an amplitude value of a transmission baseband signal that is input to the distortion compensation unit and multiplies the read distortion compensation coefficient by the transmission baseband signal. The distortion compensation coefficients stored in the distortion compensation table are sequentially updated such that an error between both the signals obtained by comparing a transmission baseband signal that is used as the reference signal with a signal that is output from the PA and is fed back (hereinafter, sometimes referred to as a “feedback signal”) is the minimum.
Patent Document 1: Japanese Laid-open Patent Publication No. 2011-199428
Before an update of the distortion compensation coefficients, i.e., before comparing a transmission baseband signal used as the reference signal with a feedback signal, an adjustment process that corrects a phase difference between both the signals and matches the phases is performed. In this phase adjustment process, a cumulative value is calculated related to each of a sample group that is associated with a transmission baseband signal (i.e., an input signal of an amplifier) and a sample group that is associated with a feedback signal (i.e., an output signal of the amplifier) and then the phase difference between both the signals is calculated by using the obtained cumulative value. Then, in the phase adjustment process, the phase difference between both the signals is corrected by using a correction amount that is calculated from the phase difference. Because the phase characteristic of the amplifier in a device varies in accordance with the ON/OFF state of the amplifier, the phase adjustment process is sequentially performed on both the signals.
However, in an amplifier, a phenomenon called a transient response of an idling current (Idq), i.e., “Idq drift”, may sometimes be generated. The idling current (Idq) is a drain current in an idling state (i.e., a state in which no signal is input to an amplifier). If the Idq drift is generated, the input/output characteristic of the amplifier is changed and a difference between the electrical power or between the phases between a sample group that is associated with the input signal of the amplifier and a sample group that is associated with the output signal of the amplifier is increased. Consequently, in the phase adjustment process, it is difficult to calculate a phase difference between the input/output signals of the amplifier with high accuracy, as a result, it is difficult to correct the phase difference between the input/output signals of the amplifier with high accuracy. In particular, if a TDD (Time Division Duplex) system that repeats a transmission period and a reception period is used in a wireless device, because Idq drift tends to be generated immediately after the amplifier is turned ON in the transmission period, it is difficult to correct the phase difference between the input/output signals of the amplifier with high accuracy.
In contrast, it is conceivable to use a method that starts the phase adjustment process after predetermined standby time has elapsed since the time point at which the amplifier is turned ON. However, in this method, although the correction accuracy of the phase difference between the input/output signals of the amplifier is secured, the time at which the phase adjustment process is started is delayed by the time period corresponding to the standby time and, as a result, the time at which the distortion compensation coefficient is updated after the phase adjustment process may possibly be delayed.